In conventional air storage gas turbine power stations, disengaging clutches are provided at the connections between the shafts of a compressor, an electrical machine (which can be operated either as a generator or as a motor) and a gas turbine. Because of these clutches, two or all three parts of the installation can be coupled together, depending upon the selection of the type of operation. It is, for example, possible to connect the electrical machine (operated as a motor) to the compressor so to drive the latter, and to store the compressed air generated by the compressor in a reservoir, for example in a subterranean cavern. It is then also possible to supply the turbine, when the need arises, with combustion air for generating combustion gas, while the turbine is connected to the electrical machine operating as a generator, in order to generate electricity. Because it is not then necessary for the turbine to drive the compressor, the maximum turbine power can be utilised in this manner for generating electricity. This type of circuit provides minimum specific investment costs.
It is, of course, also possible to connect all three parts of the installation together, i.e. to drive the compressor and the generator by the turbine without extracting air from the cavern, hence using the complete machine set as a gas turbine group. This manner of operation does, however, require the compressor and the turbine to be designed for the same mass flow. This requirement is not necessarily a requirement for exclusively storage operation, however, because another distribution may be optimum.
The way in which such an installation can be connected in mixed gas turbine and storage operation is described in the German patent application No. P 34 11 444.0 of Mar. 28, 1984. In this arrangement, the air supply from the compressor to the combustion chamber of the gas turbine is throttled. Accordingly the combustion gas flow for the turbine is also throttled. When the installation operates at part-load, the surplus pressurised airflow supplied by the compressor is stored in the cavern. In such an installation, the generator output can be substantially increased beyond that obtainable with normal gas turbine operation by reducing the output of the compressor by, for example, guide vane adjustment or by disconnecting the compressor and extracting compressed air exclusively from the storage cavern.
In such air storage gas turbine power stations, the change from storage operations to electricity generation is rapid, involves few problems and is associated with low losses. The installation can be rapidly started from cold within a few minutes by cranking the shaft and igniting the combustion chamber.
In coal-fired air storage gas turbine power stations, which are now becoming increasingly important, the conditions are not so favorable, particularly with installations in which the coal is burnt in supercharged fluidized bed firing systems. Such fluidized bed firing systems are extremely suitable for firing coal, because they allow the sulphur content to be easily fixed by the addition of limestone and, due to the relatively low combustion temperature in the fluidized bed, they produce fewer oxides of nitrogen and their ash does not melt. For this reason, there are no slag particles (which have a particularly erosive effect on the turbine blading) in the ash particles still present after filtration of the exhaust gas. Accordingly, the fluidized bed firing system substantially increase the operational life of the turbines.